This invention relates to magnetic flaw detecting method and apparatus for inspecting inner surface of steel pipes, especially suitable for the inspection of elongated steel pipes with relatively small diameters.
The present inventors have long been engaged in the research and development of the magnetic flaw detectors for detecting flaws present on the outer surfaces of slubs or round steel bars or flaws present on the outer or inner surfaces of steel pipes. The known magnetic flaw detectors operate on similar principles, in which a strong magnetic field is produced by exciting magnets on a surface to be inspected and a magneto-sensitive element is placed in the magnetic field thus produced to detect a magnetic flux which leaks from a flaw which may present on the surface under inspection.
For inspecting inner surfaces of a steel pipe, there have been known two types of magnetic flaw detection systems, i.e. an internal type in which a head assembly mounting a magnetizer and a detector is inserted into the steel pipe and moved along the inner surfaces thereof and an external type in which a similar head assembly is moved along the outer surfaces of the pipe. However, the prior art magnetic flaw detection methods and apparatus of this sort have inherent drawbacks. More particularly, the internal type in which the head assembly has to be inserted into the pipe is limited to those pipes which have a large diameter (over 300 mm in inside diameter, while the external type is applicable to pipes of relatively small diameters but poor in detecting ability.
It is therefore a primary object of the present invention to provide a magnetic flaw detector which is capable of inspecting small-diameter and thick-walled pipes for detecting flaws on the entire area of the inner wall surfaces of the pipes with high detecting efficiency and ability.
It is another object of the present invention to provide a magnetic flaw detector which can follow undulations on the inner surfaces of a pipe to perform flaw detection of high reliability and precision.
It is still another object of the present invention to provide a magnetic flaw detector which can scan the surfaces to be inspected in a reliable manner and stable conditions.